1. Field of the Invention
This invention relates to molds of curved, vertical and horizontal continuous casters for casting slabs, blooms and billets and continuous casting processes using said molds, and more particularly to molds and continuous casting processes that prevent the occurrence of breakout and produce very clean castings free of oscillation marks, surface and other defects.
2. Description of the Prior Art
Molten steel or other molten metal poured into a mold of a continuous caster leaves it as hot cast product after cooling and solidifying with the extraction of heat therefrom. FIG. 1 shows how a solidifying shell is formed and grows. Molten metal 5 is poured into a mold 1 where cooling water passed through cooling water piping 4 contained in the mold cools the molten metal by removing heat therefrom. Then, a solidifying shell 7 is formed and grows where the metal contacts the inner wall of the mold 1. A powder 18 sprinkled over the molten metal 5 protects its surface from an oxidizing atmosphere. Infiltrating between the inner wall of the mold 1 and the solidifying shell 7 as a part of slag 19, the powder 18 serves as a lubricant to prevent the sticking of the solidifying shell 7. The shell 7 solidifies and contracts as it descends through the mold 1 while forming localized air gaps between itself and the inner wall of the mold as a result of the bulging of the shell 7 caused by the recuperative action thereof until the leaving of a cast product therefrom.
When the powder 18 is used in continuous casting, the mold 1 is oscillated so that the powder 18 is fed along the inner wall of the mold 1. But this oscillation leaves oscillation marks on the solidifying shell 7 and causes other surface defects by entrapping the powder 18 therein.
There are some conventional continuous-caster molds that have ceramics and other materials of low heat conductivity affixed to the inner wall thereof. For example, molds 1 proposed in Japanese Provisional Patent Publications Nos. 173061 of 1983 and 195742 of 1986 have such materials affixed from the upper end to the lower end or middle thereof, including the point where solidification of molten metal starts, with a view to slowly cooling the molten metal 5 or the solidifying shell 7. Also, Japanese Provisional Patent Publication No. 13445 of 1983 proposes a mold 1 which has such wean-resistant materials as ceramics and stainless steel affixed to the inner wall thereof, including the vicinity of the lower end thereof, in order to prolong the mold life.
In the molds proposed in Japanese Provisional Patent Publication Nos. 173061 of 1983 and 195742 of 19086, solidification starts at the surface of the molten metal. Therefore, the need for the powder 18 and, as a consequence, the problems of oscillation marks and powder entrapment remain unremoved. On the other hand, the wear-resistant materials disclosed in Japanese Provisional Patent Publication No. 13445 of 1983, which are used to protect the lower end of the molds used in atmospheres of very high temperatures, have no effect on the solidification of the poured molten metal. Accordingly, the problems of oscillation marks and powder entrapment again remain unsolved.
For the affixing of ceramics to the surface of other substance, Japanese Provisional Patent Publication No. 93474 of 1989 discloses a method in which a layer of fine particles or fine powder of substances, which are strongly reactive and adhesive to ceramics and the substance to which the ceramics are affixed and whose particle size is smaller than the roughness of the surfaces to be joined together, and whose thickness is larger than the surface roughness, is inserted between them, with adhesion accomplished by subsequent application of pressure and heat. Japanese Provisional Patent Publication No. 120579 of 1983 discloses a method of joining such inorganic substances as ceramics and glass to such metals as platinum and copper. In this method, a paste containing 20 to 80 percent by weight of a powder of the inorganic material and 80 to 20 percent by weight of a powder of the metal to be joined together is applied to both materials which are then joined together by the application of heat.
But the conventional joining methods involving the application of pressure and heat are unsuitable for use on continuous-caster molds because they are too large to assure uniform heating. In a mold in which metal and ceramics are joined together, the ceramics are in contact with molten metal and the metal with cooling water, whereby a temperature difference arises therebetween. Because there is a considerable difference between the coefficients of linear expansion of the metal, inorganic adhesive and ceramics, the inorganic adhesive that cannot absorbs thermal stress causes cracks and nicks at joint boundaries, during the casting operation in which the mold is repeatedly exposed to heat, thereby lowering the adhesive strength and creating a danger of peeling. Inorganic adhesives mixed with metal powder also involve the danger of cracking and peeling resulting from the difference in their coefficients of linear expansion. Conventional adhesives, in addition, do not have high enough heat conductivity to permit sufficient heat extraction between the mold and molten metal and, therefore, do not permit the formation of adequately thick and stable solidified shells. To prevent breakouts, as a consequence, it becomes necessary to lower the casting speed, which results in the lowering of productivity. If the thickness of the ceramics is reduced to achieve the extraction of a greater amount of heat, a decrease in mechanical strength and the shortening of mold life through wearing may result.